1. Technical Field
The present invention relates to a method for molding a molded foam for use in, e.g., a climate control duct for vehicles, and such a molded foam.
2. Related Art
A conventional molded foam molding method involves melting and kneading a foaming agent and a base resin by an extruder, extruding the blend through a die to mold the blend into a foamed parison in a cylinder shape, clamping split mold blocks with the parison inserted therebetween, and subjecting the parison to blow molding by blowing a pressurized gas into the parison (refer to Patent Document 1). Herein, a polypropylene-based resin is used as the base resin in consideration of heat resistance, rigidity and the like regarding the molded body.
Such a conventional technique has the following problem. That is, when the foamed parison of the polypropylene-based resin is extruded, foamed cells that are open to the atmosphere rapidly expand. Consequently, surfaces of the cells become broken and an expansion ratio adversely decreases.
In this regard, according to a molding method disclosed in Patent Document 2, it is possible to obtain a molded foam with a high expansion ratio by using a polypropylene-based foaming resin with an improved melt tension. The molding method in Patent Document 2 introduces a long chain branching structure in order to improve the melt tension of the polypropylene-based resin.
In the foaming and blow molding in Patent Document 2, however, the polypropylene having a long chain branching structure causes the following problem. That is, if the expansion ratio increases, a pinhole may be formed with ease in a portion where a foamed resin expands largely (e.g., a portion where a blow ratio is high) at the time of molding.
In addition, the molded foam using the polypropylene having a long chain branching structure has a problem of considerably deteriorated impact resistance.
Because of the circumstances described above, it has conventionally been difficult to mold a molded body which is made of a foamed resin expanding largely at the time of molding and which has a high expansion ratio. Therefore, it has been difficult to obtain a molded foam which has a portion where a foamed resin expands largely in molding, includes no pinhole, is reduced in weight, and is excellent in heat insulating property and impact resistance.
The documents that describe the related art are listed below.
Patent Document 1: JP 63-309434 A
Patent Document 2: Japanese Patent No. 3745960